The metabolism of chromate by rat liver microsomes has been shown to involve the electron transport-Cytochrome P-450 system. Specific form(s) of Cytochrome P-450 has been shown to reduce chromate to chromium (III). Metyrapone, carbon monoxide, 3-pyridinealdehyde-NAD and 2'-AMP inhibited the rate of reduction of chromate by microsomes and NADPH, whereas high concentrations of dimethylsulfoxide enhanced the rate. The chromium(III) generated upon microsomal reduction of chromate formed a complex with NADP ion. Microsomes were shown to inhibit the solubility of nickel subsulfide but enhance the amount of nickel bound to DNA.